In devices for providing power through a relay contact point to a load circuit, such as an electric heater, one major circuit fault type is the welding of the relay contact point. For example, in a floor panel heating system that controls the supply of power to a floor heating panel that contains an electric heater, the supply of power to the electric heater is controlled in order to obtain the target temperature in the floor heating panel through controlling the relay contact point based on the temperature of the surface of the floor heating panel. Consequently, if there is welding of the relay contact point due to, for example, breakdown of the contact point, then it will be necessary to stop the supply of power to the electric heater, and necessary to detect accurately this type of circuit fault.
Conventionally, there have been proposals for a technology, as a technology for detecting this type of circuit fault, for detecting welding of the relay contact point through detecting, using a photocoupler, whether or not there is leakage current that flows in the relay contact point in a state wherein the relay contact points are open, doing so by connecting the photocoupler to both sides of the relay contact point. (See, for example, Japanese Unexamined Patent Application Publication 2009-168404.)
However, in this type of conventional technology, relay faults have been detected as one type of circuit fault, and there has been a problem in that it has not been possible to perform detection accurately for an insulation fault in the load circuit.
For example, in a floor heating panel system, when the floor heating panel is installed there may be inadequate insulation resistance between the electric heater and the ground voltage (the earth), due to, for example, damage to the electric heater that occurs during the installation work, precluding electrical safety. Because of this, it is necessary to detect with accuracy insulation faults in the load circuit.
The present invention is to solve this type of problem, and the object thereof is to provide a circuit fault detecting technology able to detect accurately an insulation fault in a load circuit.